Hallucination, what is it? Free wandering of the mind, the ability to see parallel universes, a soul’s flight through a continuum of variants, or just a brain malfunction? Is it a disease or a normal physiological reaction to a specific stimulus or set of stimuli?

Science defines hallucination as a sensory experience of something that does not exist outside the mind. Hallucinations can affect any senses (vision, hearing, smell, taste, and tactile feelings) and bodily sensations.

We think of hallucinations as something rare, but recent studies have shown that about 40% of people reported hallucinating at least once in their life, and about 7% reported them at least once a month, about 3% – once a week, and 2.4% – more than once a week.

These numbers, more likely, are still underestimated because people are thinking of hallucinations as a bad thing and associate them with being mentally ill, so do not admit experiencing them. Just a single example: do you ever experience the sensation of crawling insects over your whole body after finding a tick on your dog? Do you count this as hallucinating? Apparently, you should.

Doctors and scientists classify hallucinations according to the senses involved in deliria:

Visual, when people are seeing objects, people, flashing or still lights, patterns;

Olfactory, when people smell odors and scents no one can smell;

Auditory, when people hear things. Auditory hallucinations could be verbal when person hears someone is speaking to him/her or simple sounds when person hears noises such as door or floor squeaking, steps etc;

Tactile, when people feel touch or movement on part of their body;

Bodily hallucinations, involving multiple senses and body parts, when people are feeling something which is not happening to them but appears as real. An example of bodily hallucinations is sexual hallucinations in patients after anesthesia or in elderly socially isolated people.

Hallucinations also can be either chronic or temporary.

In medical terms, hallucinations are caused by a variety of factors. These factors include mental illness, the side effects of medications, physical illnesses like epilepsy or alcoholism, anesthesia, substance and alcohol abuse, prolonged sleep deprivation, terminal illnesses, such as brain cancer, kidney and liver failure, or AIDS, high fevers, migraine headaches, social isolation, seizures, deafness, blindness, vision problems, stress, bereavement, depression and dementia.

It is interesting to note that for many mental and psychiatric disorders the presence of hallucinations is a primary symptom for diagnosis. Overall, different diseases are linked to different hallucination types. For example, in schizophrenia the auditory verbal hallucinations are prevalent compared to dementia, Parkinson’s and Alzheimer’s diseases where visual hallucinations are more commonly observed.

Thus, most of the studies aimed to reveal mechanisms for hallucination phenomena were carried out in psychiatric patients. Recent developments in the visualization technique, along with the developments in visual and automated, data-driven classification methods, helped to link the hallucination phenomenon with morphological differences in specific areas of brain.

There are multiple findings reported, often conflicting and not clearly explained. Here we will concentrate on just a few current models that attempt to shed the light on the physiological mechanisms of hallucinations.

Auditory verbal hallucinations were extensively studied. Various brain morphology changes were reported in schizophrenia patients who are hallucinating. The findings point to the differences in gyri, particularly in some specific regions. One recent article points to the parasingulate sulcus in the medial prefrontal cortex. This sulcus develops late during the fetal growth and reported as being involved in the discrimination between real and imaginary information thus helping in providing the best judgment.

Scientists reported the increased volume of grey matter in the region of medial prefrontal cortex but gyri status is reduced and paracingulate sulcus (gyrus) length is decreased in some schizophrenia patients. Comparison between schizophrenia patients with and without hallucinations showed that approximately 1 cm reduction in sulcal length is associated with significant increase in reported hallucination.

The studies of visual hallucinations also concentrated on revealing the changes in brain morphology. Various differences in grey matter (usually atrophy, but sometimes hypertrophy) were identified in occipital, parietal, temporal and frontal lobes, as well as in some subcortical structures in patients with schizophrenia and neurodegenerative conditions. The analysis of existing data helped to describe the cortical signature for visual hallucination susceptibility. The findings confirm that functional neurophysiological events are involved in hallucinations along with structural changes.

The neural mechanism of hallucinations

This brings our attention to various hypotheses about neural mechanism of hallucinations. It would be fair to say that when scientists have too many hypotheses attempting to explain the same phenomenon, the actual understanding of this phenomenon tends to be rather limited. As data accumulate, the number of hypotheses grows. As the most studied type of hallucinations, auditory verbal hallucinations are described by multiple models developed over the years: cognitive-based top-down model, sensory-based bottom-up model, combined bottom-up and top-down models, social hypotheses – to name just a few.

Most recent model, named the neurophenomenal resting state hypothesis considers the influence of the brain’s resting state activity in various neural networks. When scientists started to study the activity of brain at rest, the initial assumption was that this activity is quite low. Well, not exactly: it turned out that the resting brain is just 5-10% less active compared to the brain performing an active task.

Neurophenomenal resting state hypothesis is based on an assumption that in hallucinating subjects the fluctuations of resting state activity are either too large, or this activity is not restored to the normal physiological level in specific network and remains quite high in the resting state. The resting state level in these cases may be as large as the change in the activity normally induced by external stimuli. This high activity level can be treated as if induced by an external stimulus, which in turn enables and predisposes to perceive this activity as hallucination. In healthy subjects, the intrinsic spontaneous fluctuation are not as large and are hence treated and coded as resting state fluctuations rather than as stimulus-induced activity. This enables the external stimuli to induce sufficient changes in neural activity distinctly different from the level of the resting state activity.

The hypothesis gives more cohesive view on the nature of hallucinations bringing together morphological, neuronal and phenomenal concepts. It still needs to be clearly proven experimentally. This whole area of research is very exciting, but still remains in infancy.

Viatcheslav Wlassoff, PhD, is a scientific and medical consultant with experience in pharmaceutical and genetic research. He has an extensive publication history on various topics related to medical sciences. He worked at several leading academic institutions around the globe (Cambridge University (UK), University of New South Wales (Australia), National Institute of Genetics (Japan). Dr. Wlassoff runs consulting service specialized on preparation of scientific publications, medical and scientific writing and editing (Scientific Biomedical Consulting Services).

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